Friday, April 23, 2010

satellite communication

ABSTRACT:

Not so long ago, satellites were exotic, top-secret devices. They were used primarily in a military capacity, for activities such as navigation and espionage. Now they are an essential part of our daily lives. We see and recognize their use in weather reports, television transmission by DIRECTV and the DISH Network, in everyday telephone calls,in GPS(global positioning system)and by the media in transmitting their text and images to multiple printing sites for speed local distribution.

In the context of a worldwide communications network, satellite communications systems are very important. Satellite communications links add capacity to existing communications capabilities and provide additional alternate routings for communications traffic. Satellite links, as one of several kinds of long-distance links, interconnect switching centers located strategically around the world. They are part of the defense communication systems (DCS) network. One important aspect of the satellite communications network is that it continues in operation under conditions that sometimes render other methods of communications inoperable. Because of this, satellites make a significant contribution to improved reliability of Navy communications.

When satellite television first hit the market in the early 1990s, home dishes were expensive metal units that took up a huge chunk of yard space. In these early years, only the most die-hard TV fans would go through all the hassle and expense of putting in their own dish. Satellite TV was a lot harder to get than broadcast and CABLE AND TV.Today, you see compact satellite dishes perched on rooftops all over the United States. Drive through rural areas beyond the reach of the cable companies, and you'll find dishes on just about every house. The major satellite TV companies are luring in more consumers every day with movies, sporting events and news from around the world and the promise of movie-quality picture and sound.Satellite TV offers many solutions to broadcast and cable TV problems. Though satellite TV technology is still evolving, it has already become a popular choice for many TV viewers.

WHAT IS A SATELLITE AND IT’S WORKING:

A satellite is basically any object that revolves around a planet in a circular or elliptical path. The moon is Earth's original, natural satellite, and there are many manmade (artificial) satellites, usually closer to Earth.• The path a satellite follows is an orbit. In the orbit, the farthest point from Earth is the apogee, and the nearest point is the perigee.• Artificial satellites generally are not mass-produced. Most satellites are custom built to perform their intended functions. Exceptions include the GPS satellites (with over 20 copies in orbit) and the Iridium satellites (with over 60 copies in orbit).• Approximately 23,000 items of space junk-objects large enough to track with radar that were inadvertently placed in orbit or have outlived their usefulness -- are floating above Earth. The actual number varies depending on which agency is counting. Payloads that go into the wrong orbit, satellites with run-down batteries, and leftover rocket boosters all contribute to the count. This online catalogs of satellite has almost 26,000 entries!Although anything that is in orbit around Earth is technically a satellite, the term "satellite" is typically used to describe a useful object placed in orbit purposely to perform some specific mission or task. We commonly hear about weather satellites, communication satellites and scientific satellite

• THE SOVIET SPUTNIK SATELLITE WAS THE FIRST TO ORBIT EARTH, LAUNCHED ONOCTOBER 4, 1957.

TYPES OF SATELLITES:

Satellites come in all shapes and sizes and play a variety of roles. For example:• Weather satellites help meteorologists predict the weather or see what's happening at the moment. Typical weather satellites include the TIROS, COSMOS and GOES satellites. The satellites generally contain cameras that can return photos of Earth's weather, either from fixed geostationary positions or from polar orbits.• Communications satellites allow telephone and data conversations to be relayed through the satellite. Typical communications satellites include Telstar and Intelsat. The most important feature of a communications satellite is the transponder -- a radio that receives a conversation at one frequency and then amplifies it and retransmits it back to Earth on another frequency. A satellite normally contains hundreds or thousands of transponders. Communications satellites are usually geosynchronous.• Broadcast satellites broadcast television signals from one point to another (similar to communications satellites).• Scientific satellites perform a variety of scientific missions. The Hubble Space telescope is the most famous scientific satellite, but there are many others looking at everything from sun spots to gamma rays.• Navigational satellites help ships and planes navigate. The most famous are the GPS NAVSTAR satellites.• Rescue satellites respond to radio distress signals .• Earth observation satellites observe the planet for changes in everything from temperature to forestation to ice-sheet coverage. The most famous are the LANDSAT series.

SATELLITE TELEVISION-A MERGE:

Why do we have satellite television? What's wrong with the terrestrial broadcasts that we have enjoyed for five decades or more? Well the real benefit lies in the fact that you get more channels. You get more channels because satellite broadcasts are able to utilise more bandwidth than conventional terrestrial systems and can therefore fit more separate channels into the space allowed.

Sattellite television really becomes important in areas where it is not possible to install cable and the broadcast television reception is poor. Both systems use radio wave signals to transmit and the waves travel in straight lines. That means for broadcast television, which is transmitted and received via land-based antenna, that the natural curvature of the earth will eventually break the signals' line of sight. It also means that other land based signals and obstructions are likely to interfere with the TV signal and cause some distortion.

Satellites that transmit TV are placed in orbit over 22,000 miles above the Earth and they rotate around the planet once every 24 hours and in the same direction that the Earth is rotating in. Because the Earth rotates once every 24 hours and the satellite moves around the Earth at the same velocity in the same direction, the satellite always stays over the same point on the surface. The satellites are said to be in geostationary orbit. Because they are high in the sky a satellite beams signals over a wide area of the planets surface. The satellites are also in communication with each other so that they can relay the same broadcast to all satellites in orbit over different parts of the world. This is how we are able to get TV live from anywhere on the Earth and view it at the same time as everyone else on the planet receiving a signal from a satellite. When you think about it it's a pretty amazing feat of technology.

When satellite television was first introduced the dishes required to receive the signal were very expensive and people were able to set them up to receive programmes that were not really intended for everybody to see. Nowadays people tend to subscribe to a 'Direct Broadcast Satellite' (DBS) provider such as DirecTV or Dish Network in the US and Sky in the UK. These providers are able to select programs and broadcast them to people as set packages, for example the 'Family', 'Sports' or 'Movie' packages. Cost will depend on which one you choose to buy.

SATELLITE TV SYSTEM:

Today, most satellite TV customers get their programming through a direct broadcast satellite (DBS) provider, such as DirecTV or DISH Network. The provider selects programs and broadcasts them to subscribers as a set package. Basically, the provider's goal is to bring dozens or even hundreds of channels to your TV in a form that approximates the competition, cable TV.Unlike earlier programming, the provider's broadcast is completely digital, which means it has much better picture and sound quality. Early satellite television was broadcast in C-band radio -- radio in the 3.7-gigahertz (GHz) to 6.4-GHz frequency range. Digital broadcast satellite transmits programming in the Ku frequency range (11.7 GHz to 14.5 GHz ).

The Components:

There are five major components involved in a direct to home (DTH) or direct broadcasting (DBS) satellite system: the programming source, the broadcast center, the satellite, the satellite dish and the receiver.• Programming sources are simply the channels that provide programming for broadcast. The provider doesn't create original programming itself; it pays other companies (HBO, for example, or ESPN) for the right to broadcast their content via satellite. In this way, the provider is kind of like a broker between you and the actual programming sources. (Cable TV companies work on the same principle.)• The broadcast center is the central hub of the system. At the broadcast center, the TV provider receives signals from various programming sources and beams a broadcast signal to satellites in geosynchronous orbit.• The satellites receive the signals from the broadcast station and rebroadcast them to Earth.• The viewer's dish picks up the signal from the satellite (or multiple satellites in the same part of the sky) and passes it on to the receiver in the viewer's house.• The receiver processes the signal and passes it on to a standard TV.

SATELLITE TV PROGRAMMING:

Satellite TV providers get programming from two major sources: national turnaround channels (such as HBO, ESPN and CNN) and various local channels (the ABC, CBS, Fox, NBC and PBS affiliates in a particular area). Most of the turnaround channels also provide programming for cable TV, and the local channels typically broadcast their programming over the airwaves.Turnaround channels usually have a distribution center that beams their programming to a geosynchronous satellite. The broadcast center uses large satellite dishes to pick up these analog and digital signals from several sources.Most local stations don't transmit their programming to satellites, so the provider has to get it another way. If the provider includes local programming in a particular area, it will have a small local facility consisting of a few racks of communications equipment. The equipment receives local signals directly from the broadcaster through fiber-optic cable or an antenna and then transmits them to the central broadcast center.The broadcast center converts all of this programming into a high-quality, uncompressed digital stream. At this point, the stream contains a vast quantity of data -- about 270 megabits per second (Mbps) for each channel. In order to transmit the signal from there, the broadcast center has to compress it. Otherwise, it would be too big for the satellite to handle. In the next section, we'll find out how the signal is compressed

Satellite TV Signal:Satellite signals have a pretty long path to follow before they appear on your TV screen in the form of your favorite TV show. Because satellite signals contain such high-quality digital data, it would be impossible to transmit them without compression. Compression simply means that unnecessary or repetitive information is removed from the signal before it is transmitted. The signal is reconstructed after transmission.

Standards of Compression:

Satellite TV uses a special type of video file compression standardized by the Moving Picture Experts Group (MPEG). With MPEG compression, the provider is able to transmit significantly more channels. There are currently five of these MPEG standards, each serving a different purpose. DirecTV and DISH Network, the two major satellite TV providers in the United States, once used MPEG-2, which is still used to store movies on DVDs and for digital cable television (DTV). With MPEG-2, the TV provider can reduce the 270-Mbps stream to about 5 or 10 Mbps (depending on the type of programming).Now, DirecTV and DISH Network use MPEG-4 compression. Because MPEG-4 was originally designed for streaming video in small-screen media like computers, it can encode more efficiently and provide a greater bandwidth than MPEG-2. MPEG-2 remains the official standard for digital TV compression, but it is better equipped to analyze static images, like those you see on a talk show or newscast, than moving, dynamic images. MPEG-4 can produce a better picture of dynamic images through use of spatial (space) and temporal (time) compression. This is why satellite TV using MPEG-4 compression provides high definition of quickly-moving objects that constantly change place and direction on the screen, like in a basketball game.

Satellite TV Encoding and Encryption:At the broadcast center, the high-quality digital stream of video goes through an MPEG encoder, which converts the programming to MPEG-4 video of the correct size and format for the satellite receiver in your house.Encoding works in conjunction with compression to analyze each video frame and eliminate redundant or irrelevant data and extrapolate information from other frames. This process reduces the overall size of the file. Each frame can be encoded in one of three ways:• As an intraframe, which contains the complete image data for that frame. This method provides the least compression.• As a predicted frame, which contains just enough information to tell the satellite receiver how to display the frame based on the most recently displayed intraframe or predicted frame. A predicted frame contains only data that explains how the picture has changed from the previous frame.• As a bidirectional frame, which displays information from the surrounding intraframe or predicted frames. Using data from the closest surrounding frames, the receiver interpolates the position and color of each pixel.This process occasionally produces artifacts -- glitches in the video image. One artifact is macroblocking, in which the fluid picture temporarily dissolves into blocks. Macroblocking is often mistakenly called pixilating, a technically incorrect term which has been accepted as slang for this annoying artifact. Graphic artists and video editors use "pixilating" more accurately to refer to the distortion of an image. There really are pixels on your TV screen, but they're too small for your human eye to perceive them individually -- they're tiny squares of video data that make up the image you see.

Satellite Dish:When the signal reaches the viewer's house, it is captured by the satellite dish. A satellite dish is just a special kind of antenna designed to focus on a specific broadcast source. The standard dish consists of a parabolic (bowl-shaped) surface and a central feed horn. To transmit a signal, a controller sends it through the horn, and the dish focuses the signal into a relatively narrow beam.

The curved dish reflects energy from the feed horn, generating a narrow beam.

The dish on the receiving end can't transmit information; it can only receive it. The receiving dish works in the exact opposite way of the transmitter. When a beam hits the curved dish, the parabola shape reflects the radio signal inward onto a particular point, just like a concave mirror focuses light onto a particular point.

The curved dish focuses incoming radio waves onto the feed horn.

In this case, the point is the dish's feed horn, which passes the signal on to the receiving equipment. In an ideal setup, there aren't any major obstacles between the satellite and the dish, so the dish receives a clear signal.In some systems, the dish needs to pick up signals from two or more satellites at the same time. The satellites may be close enough together that a regular dish with a single horn can pick up signals from both. This compromises quality somewhat, because the dish isn't aimed directly at one or more of the satellites. A new dish design uses two or more horns to pick up different satellite signals. As the beams from different satellites hit the curved dish, they reflect at different angles so that one beam hits one of the horns and another beam hits a different horn.The central element in the feed horn is the low noise blockdown converter, or LNB. The LNB amplifies the radio signal bouncing off the dish and filters out the noise (radio signals not carrying programming). The LNB passes the amplified, filtered signal to the satellite receiver inside the viewer's house.

Photo courtesy DirecTV

Satellite Receiver:The end component in the entire satellite TV system is the receiver. The receiver has four essential jobs:• It de-scrambles the encrypted signal. In order to unlock the signal, the receiver needs the proper decoder chip for that programming package. The provider can communicate with the chip, via the satellite signal, to make necessary adjustments to its decoding programs. The provider may occasionally send signals that disrupt illegal de-scramblers as an electronic counter measure (ECM) against illegal users.• It takes the digital MPEG-2 or MPEG-4 signal and converts it into an analog format that a standard television can recognize. In the United States, receivers convert the digital signal to the analog National Television Systems Committee (NTSC) format. Some dish and receiver setups can also output an HDTV signal.• It extracts the individual channels from the larger satellite signal. When you change the channel on the receiver, it sends just the signal for that channel to your TV. Since the receiver spits out only one channel at a time, you can't tape one program and watch another. You also can't watch two different programs on two TVs hooked up to the same receiver. In order to do these things, which are standard on conventional cable, you need to buy an additional receiver.• It keeps track of pay-per-view programs and periodically phones a computer at the provider's headquarters to communicate billing information.Receivers have a number of other features as well. They pick up a programming schedule signal from the provider and present this information in an onscreen programming guide. Many receivers have parental lock-out options, and some have built-in digital video recorders(DVRs), which let you pause live television or record it on a hard drive.These receiver features are just added bonuses to the technology of satellite TV. With its movie-quality picture and sound, satellite TV is becoming a popular investment for

consumers. Digital cable, which also has improved picture quality and extended channel selection, has proven to be the fiercest competitor to satellite providers. The TV war is raging strong between satellite and digital cable technologies as well as between the providers who offer these services. Once considered luxuries in most households, satellite and digital cable are becoming quite common as providers bundle TV with Internet and phone services to offer competitive deals and win over customers.

Problems with Broadcast TV:Conceptually, satellite TV is a lot like broadcast TV. It's a wireless system for delivering television programming directly to a viewer's house. Both broadcast television and satellite stations transmit programming via a radio signal.Broadcast stations use a powerful antenna to transmit radio waves to the surrounding area. Viewers can pick up the signal with a much smaller antenna. The main limitation of broadcast TV is range. The radio signals used to broadcast television shoot out from the broadcast antenna in a straight line. In order to receive these signals, you have to be in the direct line of sight of the antenna. Small obstacles like trees or small buildings aren't a problem; but a big obstacle, such as the Earth, will reflect these radio waves.If the Earth were perfectly flat, you could pick up broadcast TV thousands of miles from the source. But because the planet is curved, it eventually breaks the signal's line of sight. The other problem with broadcast TV is that the signal is often distorted, even in the viewing area. To get a perfectly clear signal like you find on cable, you have to be pretty close to the broadcast antenna without too many obstacles in the way.The Satellite TV Solution:

Satellite TV solves the problems of range and distortion by transmitting broadcast signals from satellites orbiting the Earth. Since satellites are high in the sky, there are a lot more customers in the line of sight. Satellite TV systems transmit and receive radio signals using specialized antennas called satellite dishes.

Satellites are higher in the sky than TV antennas, so they have a much larger line of sight range.

The TV satellites are all in geosynchronous orbit, meaning that they stay in one place in the sky relative to the Earth. Each satellite is launched into space at about 7,000 mph (11,000 kph), reaching approximately 22,200 miles (35,700 km) above the Earth. At this speed and altitude, the satellite will revolve around the planet once every 24 hours -- the same period of time it takes the Earth to make one full rotation. In other words, the satellite keeps pace with our moving planet exactly. This way, you only have to direct the dish at the satellite once, and from then on it picks up the signal without adjustment, at least when everything works right.At the core, this is all there is to satellite TV. But as we'll see in the next section, there are several important steps between the original programming source and your TV set.

ADVANTAGES OF SATELLITE COMMUNICATIONS:. Satellite links are unaffected by the propagation variations that interfere with hf radio. They are also free from the high attenuation of wire or cable facilities and are capable of spanning long distances. The numerous repeater stations required for line-of-sight or troposcatter links are no longer needed. They furnish the reliability and flexibility of service that is needed to support a military operation.Capacity:The present military communications satellite system is capable of communications between backpack, airborne, and shipboard terminals. The system is capable of handling thousands of communications channels.Reliability:Communications satellite frequencies are not dependent upon reflection or refraction and are affected only slightly by atmospheric phenomena. The reliability of satellite communications systems is limited only by the equipment reliability and the skill of operating and maintenance personnel.Vulnerability:Destruction of an orbiting vehicle by an enemy is possible. However, destruction of a single communications satellite would be quite difficult and expensive. The cost would be excessive compared to the tactical advantage gained. It would be particularly difficult to destroy an entire multiple-satellite system such as the twenty-six random-orbit satellite system currently in use. The earth terminals offer a more attractive target for physical destruction. These can be protected by the same measures that are taken to protect other vital installations.A high degree of freedom from jamming damage is provided by the highly directional antennas at the earth terminals. The wide bandwidth system that can accommodate sophisticated anti-jam modulation techniques also lessens vulnerabilityFlexibilityMost operational military satellite earth terminals are housed in transportable vans. These can be loaded into cargo planes and flown to remote areas. With trained crews these terminals can be put into operation in a matter of hours. Worldwide communications can be established quickly to remote areas nearly anywhere in the free world.

IT’S FUTURE-THE CONCLUSION:

In the future SATELLITE TV will open up so many possibilities that it is hard to imagine what our lives will be like in 30, or even just 20 years from now. To give an idea of how fast things are going. 50 years ago, there was nothing in space that was made by humans. Now there are even satellite graveyards (specific orbits where obsolete satellites are "parked"). The possibilities of satellite TV technology are growing faster every year. What took 10 years to develop 30 years ago is now done in 2 years.

Satellite TV is one of the driving forces for satellite technologies because the need to please million of subscribers is much stronger than the need to please the relative limited needs of communications for commercial purposes. The future of satellite TV is so bright, that a supernova would pale in comparison.